The present invention relates to a method and a system for through fault detection in electrical devices.
In many electric devices, such as power transformers, bus-bars, generators and others, a differential protection technique is used by a protective relaying system, sometimes within an Intelligent Electronic Device (I.E.D.), to determine if the electric device has suffered an internal failure. The electric device is usually positioned along a power line and has a first side connected for example to a power source from which an incoming current flows into the device, and a second side through which a current flows out from the device, for example towards a load. FIG. 1A shows an example of a single phase transformer 200 operating under normal conditions. Located on a first line side 201 there is a first current transformer 202, while a second current transformer 203 is located along a second line side 204. The two current transformers 202 and 203 reduce the high currents on the power line to an appropriate level for processing by a differential protection technique which is usually part of an I.E.D.
The differential protection technique compares the electrical current differential between the source current and the load current. Under normal operating conditions, the magnitude of the differential current also referred to as the operating current (IOP) is approximately zero. IRES1 and IRES2 are the so-called restraining currents, derived from the primary currents. The bold-faced notations, like IOP, used for currents herein are for phasor current having a magnitude as well as an angle.
The differential protection technique generally utilizes the relationship between the operating current IOP and the average of the restraining currents IRES1 and IRES2 and in particular uses the comparative relationship between the operating current (IOP) and the average restraining current ((Ires1+Ires2)/2) as shown in FIG. 2. Any value for IOP above the characteristic operating curve shown in FIG. 2 indicates that a fault has occurred.
The value of the operating current IOP increases when an internal fault condition exists within an electric device but a high operating current can be also the result of external fault conditions which in turn may cause a current transformer to saturate and the operating current IOP to spike. FIG. 1B shows a single phase transformer experiencing an external fault condition. FIG. 1C illustrates a single phase transformer that has experienced an internal fault condition. Similar conditions apply for a three-phase transformer as well.
It is to be understood that internal and external faults are hereby referred to as faults which are located internally or externally with respect to the zone defined by the power device to be protected and the surrounding current transformers associated therewith.
In operation, high operating currents caused by internal fault conditions require intervention of the differential protection unit directly associated with the electric device. On the contrary, external fault conditions which are considered and usually referred to as a “through fault condition” require that no remedial actions be taken on the part of the protective logic directly protecting the device because the fault is not internal to the device.
Thus it is desirable to provide a solution which better discriminates and distinguishes between a high operating current (IOP) condition caused by a fault external to the electric device, and a high operating current (IOP) condition caused by a fault internal to the electric device.